Many types of luminescent devices exist, including a number of all solid state devices. Solid state devices are preferable over incandescent or fluorescent bulbs in that they are lighter; more compact, can be made smaller, and can have higher efficiency. Examples of solid state luminescent devices are light emitting diodes (LEDs), such as gallium arsenide or silicon carbide LEDs, organic light emitting diodes (OLEDs), such as OLED displays being marketed by Uniax Corporation and CDT Ltd., and doped zinc sulfide devices that have been marketed for a number of years, for example by GE® as Limelite™ nightlights, and American Tack and Hardware, Co. Inc., (Monsey, N.Y.) as Nitelite™ nightlights. Any of these devices can be fabricated into arrays to represent numbers or letters, or pictures.
Of the various luminescent devices and displays the OLEDs are the newest and least mature technology. OLEDs typically consist of a thin film structure comprising a transparent electrode, usually indium doped tin oxide (ITO) on a glass or plastic support layer, the ITO optionally coated with polyaniline or poly(ethylenedioxythiophene) (PEDOT), one or more organic containing layers, typically a hole conducting layer, for example, of a triphenylamine derivative, a luminescent layer, for example, a polyphenylenevinylene derivative or a polyfluorene derivative, an electron conducting layer, for example, an oxadiazole derivative, and a second electrode, for example, calcium, magnesium, aluminum, and the like.
The advantages of the OLED devices are, lightweight, potentially low cost (although this has yet to be demonstrated commercially), the ability to fabricate thin film, flexible structures, wide viewing angle, and high brightness. The disadvantages of OLEDs are short device lifetimes, increasing voltages when operated in a constant current mode, and broad spectral widths. The efficiency of OLEDs is limited by the nature of the excited state of organic molecules. Typically, both the singlet and triplet excited states are populated during the operation of an OLED. Unfortunately, only decay from the singlet state produces useful light. Decay from the triplet state to a singlet ground state is spin forbidden and therefore slow, giving non-radiative processes more time to take place. Because the triplet state is three-fold degenerate and the singlet state is not degenerate; three quarters of the excited electrons enter the triplet state and produce little or no light.
An additional disadvantage of OLEDs is the relatively short lifetime of the excited state of organic molecules. In a display application each pixel is scanned 10 to 100 times every second, typically 60 times every second. It is desirable for the light from the pixel to decay on about the same time scale. If the pixel decays too slowly each subsequent image will be scanned over the not yet faded previous image, and the image will blur. If the pixel decays too quickly, there will be a noticeable flicker.
There is a need for a solid state device that is not limited by the short lifetimes of OLEDs. The short life of OLEDs is suspected to arise from the decomposition or alteration of the organic layers during operation.
There is also a need for electroluminescent devices that have stable I-V characteristics making the associated electronics simpler.
There is also a need for electroluminescent devices with pure color characteristics that are more amenable to color displays. For color television, monitors, and the like, red, blue, and green devices with exacting color are required.
There is also a need for electroluminescent devices with higher efficiency, not limited by decay from non-luminescent triplet states.
There is also a need for electroluminescent devices with phosphorescent decay times in the appropriate range for scanned displays and passive displays.